I know I'm really late to the party here, but I'm gonna put my two cents here on a couple of things.
I completely understand the viewpoint that Littlefoot is not the angel in the first movie that he seems to have become in the sequels and I agree with that. However, after watching the movie again, I don't believe that him choosing Ducky as the bait really counts towards this. I know this is a long post, but just hear me out on this.
When you look at what everybody was supposed to be doing, it makes sense that Ducky was the one to lure Sharptooth into the water. The point was to get him into the water, and Ducky is the only swimmer in the group. Littlefoot and Spike needed to be at the boulder ready to start pushing it when Sharptooth was in the right spot, and I honestly doubt that either one of them would have been able to climb back up fast enough after going into water where neither of them are as good of swimmers. So then the response to this could be that Petrie could have been bait as well. Well he hadn't believed in himself to finally master his flying, so he wouldn't have been able to get away from Sharptooth as fast as Ducky could. That only leaves Ducky because everybody else has their respective parts and she was the only one who could pull off luring Sharptooth.
Now one may ask how Littlefoot could have known that Ducky was a good swimmer. Not once in the movie does she refer to herself as a swimmer, but instead a "big-mouth." Unless that automatically means she can swim, there really is no indication that she is one. Right at the moment the two meet, we see Ducky jumping out of the water. We can only assume that Littlefoot either had the knowledge that big-mouths were also good swimmers before meeting Ducky or Ducky explained it to him off screen at some point during their travels. The point of this, which builds off more on my previous paragraph, is that Littlefoot may have known that Ducky could have easily eluded Sharptooth in the water which is why he chose her to be the bait. Again, the other three are not swimmers, which would make Ducky the perfect choice.
Or I could be just rambling nonsense and not know what I'm talking about, which I wouldn't be surprised about because it happens on occasions
